Indexable cutting inserts are normally screwed into the body of the tool by means of clamping screws, clamped therein by means of a clamping bolt engaging the center hole by being tilted in the hole, or secured in the body of the tool by screwing and simultaneous canting of the clamping bolt.
A cutting tool is known from German Pat. No. 15 52 296 which consists of a tool holder and an exchangeable, indexable cutting insert provided with a center hole. The indexable cutting insert is capable of being clamped by way of a clamping bolt disposed in a shank borehole and capable of being screwed into a threaded borehole provided in the shank of the tool, the clamping bolt engaging the center hole of the indexable cutting insert with its head portion. The section of the shank borehole disposed adjacent to the recess for receiving the indexable cutting insert is tapered downwardly in the form of a frustum of a cone and cooperates with a section of the clamping bolt which is tapered downwardly in the form of a truncated cone. The longitudinal axis of the truncated section is displaced with respect to the longitudinal axis of the adjacent threaded borehole towards the supporting surface of the cutting insert, so that a tilting motion, and thus clamping of the indexable cutting insert, is achieved when the clamping bolt is screwed into the borehole. It is a drawback of this system that the clamping force of the indexable cutting insert is insufficient mainly in the direction of the surface supporting the cutting insert. This means that the indexable cutting insert may lift itself from the supporting surface in the presence of higher loads. This drawback is caused by the only slight relative motion of the clamping surface of the clamping bolt with respect to the wall of the center hole of the cutting plate.
German laid-open patent specification No. 24 02 971 specifies a cutting tool holder for receiving a center-hole, indexable cutting insert arranged in a recess of the supporting plate and capable of being clamped against the supporting surfaces and the supporting plate by means of a clamping bolt screwed into a threaded borehole of the shaft of the holder. In this embodiment, a conical section of the clamping bolt abuts a counter surface having the form of a bay provided on a bushing which encloses or surrounds the clamping bolt at least partially, and which is inserted in a borehole of the shaft of the holder. The preferably elastic bushing is provided with a collar supported in a mating recess of the supporting plate. When the clamping bolt is screwed in, it is deflected by the bay, and the head portion engaging the center hole clamps the indexable cutting insert. The drawback of this tool holder is that lateral loads acting on the indexable cutting insert may cause the clamping bolt to give way laterally, which may cause loosening of the clamping of the indexable cutting insert. Moreover, the fact that the bushing has an unsymmetrical structure may cause poor positioning of the bushing and thus lead to poor clamping of the cutting insert or, in the least favorable case, to no clamping of the cutting insert at all, which means that the bushing must be accurately positioned.
A cutting tool is specified in German published patent disclosure No. 27 03 696 in which a center-hole, indexable cutting insert is supported in a recess of the body of the tool on a supporting plate and capable of being clamped by means of a screwable clamping bolt of which the head portion engages the center hole of the indexable cutting insert. The clamping bolt is at least partially surrounded by a bushing inserted in a borehole of the tool body, this bushing having a collar and being supported on a mating surface of the supporting plate. On its face side, the bushing is provided with a beveling in the form of a truncated cone which is contacted unilaterally by a collar of the clamping bolt disposed directly beneath the indexable cutting insert when the clamping bolt is screwed into place. This causes the bolt to be tilted and to clamp the indexable cutting insert with respect to the supporting surface and the lateral supporting surfaces.
The disadvantage of this latter design is that the surface of the bushing contacting the collar of the clamping bolt is disposed outside of the zone of the shaft of the tool in the base plate, which means that said surface must absorb a flexural torque and thus be provided with particular stability. Moreover, it can be secured in the shaft of the tool with sufficient rigidity only by means of a press fitting, which is costly in terms of manufacturing technique.
The common feature of the systems described above is that while a indexable cutting insert must be clamped both against the supporting surface and at least one lateral supporting surface by screwing and simultaneous tilting of a clamping bolt, this system as such fails to achieve a clamping that could be deemed sufficient in general or at least sufficient simultaneously in both those directions, considering modern requirements in this regard. The reasons for such insufficient clamping are excessive frictional forces or an excessive increase of the frictional forces between the clamping bolt and the body of the tool as compared to the forces transmitted by the clamping bolt onto the indexable cutting insert, those forces occurring when said bolt is screwed into place. Furthermore, elements of design or insufficient consideration of manufacturing tolerances are responsible in individual cases for an unreliable positioning and clamping of the indexable cutting insert.
Another decisive shortcoming of the known systems is that the change in pressure forces transmitted from the clamping bolt to the walls of the center hole of the cutting insert is from constant to increasing in each case by a constant angle of rotation when the clamping bolt is screwed into place. This normally makes it impossible to gradually approach and thus safely reach with a predetermined torque of the torque wrench the possible limit value of the pressure forces acting between the clamping bolt and the indexable cutting insert.
Therefore, it is the problem of the present invention to provide a cutting tool of the type defined above, in which the described drawbacks are substantially reduced and which thus ensures satisfactory clamping of the known indexable cutting insert with a center hole both against the supporting base plate and the lateral plate support, with reduced bearing of the manufacturing tolerances on the clamping of the plate. It is the particular problem of the invention to provide measures to the effect that the increase of the pressure forces transmitted from the clamping bolt to the walls of the center hole of the cutting insert is reduced as the clamping bolt is screwed into a threaded borehole in the shaft of the tool, i.e., that the increase of these forces is degressive.